1. Field of the Invention
The invention relates to a pressure sensitive adhesive for an optical film. The invention further relates to a manufacturing method for a pressure sensitive adhesive layer for an optical film using the pressure sensitive adhesive for an optical film and a pressure sensitive adhesive for an optical film obtained by the manufacturing method. The invention still further relates to a pressure sensitive adhesion type optical film at least one surface of which the pressure sensitive adhesive layer for an optical film is laminated. The invention still further relates to an image display such as a liquid crystal display, an organic electro-luminescent (EL) display, Plasma Display Panel (PDP) and the like using the pressure sensitive adhesion type optical film. Examples of the optical film include: a polarizing plate, a retardation plate, an optical compensation film, a brightness enhancement film and a laminate thereof.
2. Description of the Related Art
A liquid crystal display indispensably requires polarizing elements disposed on both sides of a liquid crystal cell because of an image forming method adopted therein and generally polarizing plates are adhered. Besides, on a liquid crystal panel, various kinds of optical elements have been used in addition to a polarizing plate in order to improve a display quality of a display. For example, there have been used a retardation plate for coloration prevention, a viewing angle increasing film for improving a viewing angle of a liquid crystal display and a brightness enhancement film for raising a contrast of a display. The films each are collectively referred to an optical film.
A pressure sensitive adhesive is usually employed in adhering an optical film described above to a liquid crystal cell. An optical film and a liquid crystal cell or optical films are usually adhered to each other using a pressure sensitive adhesive therebetween in order to reduce a light loss. In such cases, a pressure sensitive adhesion type optical film in which a pressure sensitive adhesive is provided in advance on one surface of an optical film as a pressure sensitive adhesive layer is generally used because of a merit such as that no necessity arises for a drying step of fix the optical film.
An optical film described above is easy to contract or expand in conditions such as heating or humidifying; therefore, after the optical film is adhered to a liquid crystal panel, film lifting or peeling off occurs with ease. In applications of vehicle installation such as car navigation and a large size television, for which a high durability is required, a necessity arises for a pressure sensitive adhesive with difficulty for causing film lifting or peeling off. Moreover, in some case, inconvenient phenomena occur that a liquid crystal cell is bowed due to contraction or expansion of an optical film, as described above, and light leaks in the peripheral area of a liquid crystal panel generated by a residual stress in the optical film proper. In order to eliminate the phenomena, a proposal has been offered on a pressure sensitive adhesive composition containing a component of a plasticizer or an oligomer as a pressure sensitive adhesive for an optical film (JP-A Nos. 9-84593 and 10-279907).
On the other hand, a pressure sensitive adhesion type optical film described above is punched or slit into pieces with a predetermined size, in which working possibilities arise that a pressure sensitive adhesive is taken away by a cutting blade or a pressure sensitive adhesive is swelled out from a cutting surface. An unfavorable possibility is expected that a pressure sensitive adhesive is taken away from or contaminates a punched optical film during visual inspection or transport of the punched optical film. It is an important issue to improve handling ability in an aspect of fabrication process of an optical film in addition to prevention of the peeling off and bowing and light leakage, whereas improvement on the issue cannot be expected with use of a pressure sensitive adhesive composition containing a component of a plasticizer or an oligomer.
An acrylic-based pressure sensitive adhesive is generally used as a pressure sensitive adhesive for an optical film from the standpoint of transparency and durability. The acrylic-based pressure sensitive adhesive is generally cross-linking treated. Cross-linking agents used in an acrylic-based pressure sensitive adhesive for an optical film that are known include: for example, an isocyanate compound, an epoxy compound, a glycidyl compound and a metal chelate (JP-A Nos. 8-199131 and 2003-49141).